With the rapid progress of the society towards the information and intelligence-oriented one in the recent years, uninterruptible power supplies are being widely used as power sources for backing-up computers which are key components of such society. The conditions, such as rated voltage, temperature, humidity and the like, under which the uninterruptible power supplies are operating extend over a wide range. Thus, the life of the sealed lead-acid storage batteries included in the uninterruptible power supplies depends heavily upon the service conditions. In general, the sealed lead-acid storage batteries installed in the uninterruptible power supplies are frequently used under a constantly charging state, i.e., so-called trickle charge condition. In a case of being used under the trickle charge conditions of a high temperature and a low humidity in particular, it is confirmed that the trickle life mode of the batteries is an early increase in the internal resistance of the batteries due to the decrease in the amount of the electrolyte.
In particular, since the uninterruptible power supplies usually have heat sources inside thereof and are usually installed indoor, the deteriorating mode of the batteries depends heavily upon the decrease in the amount of the electrolyte and thus the trickle life of the batteries sometime terminates early.
The material for the container and the container cover of the conventional sealed lead-acid storage batteries starts from the use of glass but at present tends towards those of resins in most cases. The containers made of a resin are indispensable for the recent trend in designing the batteries to have a smaller size and a lighter weight. However, in the case of using the resin container, the decrease in the water content cannot be avoided.
Attempts have been made in order to suppress the escape of water from the resin container in such manners of adding mica which is a substance free from the humidity transmitting property to acrylonitrile-butadiene-styrene resin (hereinafter referred to as ABS resin) which has frequently been used for the containers and the container covers of the batteries (Japanese Unexamined Patent Publication Sho 62-71166), of adding glass flakes (Japanese Unexamined Utility Model Publication Hei 6-71152), and of adding glass flakes to polyphenylene ether resins (Japanese Unexamined Patent Publication Hei 6-203814).
The most significant problem which must be solved in a sealed lead-acid storage battery which does not permit any replenishment or supplementation of water from outside of the container is to reduce to a minimum an escape of water from the inside of the container. As stated above, introduction of an additive for impairing the permeation of water from the resin advantageously increases the mechanical strength of the resin, but causes a problem that the anti-impact strength of the resin is decreased dramatically. Under the growing trend of designing the batteries to have a smaller size and a lighter weight, the decrease in the impact strength is a fatal problem in such a case of sudden drop of the batteries or the like accident.
In the sealed lead-acid storage batteries in general, container covers have ports for injecting the electrolyte and top lids which are welded to the container covers for covering vent valves provided on the ports, by means of ultrasonic welding. However, in such a case of changing the resin material for the container covers or the top lids therefor, or of adding additives thereto, the conditions for the ultrasonic welding between the resin components are also changed and there has been a tendency of seriously decreasing the welded strength.
The sealed lead-acid storage batteries for the computer backing-up application are required to be constantly in charged states. For that reason, the trickle charge is constantly performed for replenishing the lost charge due to the self-discharge by an outside power source. When the sealed lead-acid storage batteries are charged, oxygen is constantly generated from the positive electrodes inside the containers. Although most of the oxygen is safely absorbed by the negative electrode, the internal pressure inside the container is still larger than the atmospheric pressure in most cases, especially in a high temperature atmosphere such as 40.degree. C. or over, and during the trickle charge, a pressure is exerted on the vent valves in the direction of opening them.
For that reason, a stress is exerted on the top lids in the direction for stripping their parts welded to the container covers from the covers, and thus there is a need for securing a satisfactory welded strength between the top lids and the container covers. When the top lids are stripped-off from the container covers, sealing property of the vent valves is damaged, the electrolyte decreases abruptly, the negative electrode plates are deactivated by being reacted with the oxygen in the air, and the capacity of the batteries decreases abruptly.
In general, the sealed lead-acid storage batteries used in the computer backing-up application are so configured as to output a high voltage by serially connecting a plurality of monoblock batteries into assembled batteries. In such application, if only one stripping-off occurs at the welded part between the container cover and the top lid in one monoblock battery constituting the assembled batteries, besides the rapid deterioration of the performance of the one monoblock battery, the assembled batteries as a whole suffer from the capacity decrease.
In addition, the monoblock batteries whose gas-tight property has once been deteriorated in this manner abruptly increase their internal resistance, thereby to remarkably decrease the charge acceptance of the assembled batteries as a whole. In a case of still continuing the use of the assembled batteries under such a circumstance, the normal monoblock batteries whose gas-tight property has not been deteriorated also fall into a charge-deficient state. Once such circumstance is brought, there is a concern that any restoration cannot be expected by replacing only the defective monoblock batteries whose gas-tight property has been deteriorated with normal ones.